1. Field of the Invention
This invention relates to electrophotographic photosensitive members and more particularly to a photosensitive member having improved electrophotographic characteristics.
2. Description of the Prior Art
There have so far been known selenium, cadmium sulfide, zinc oxide, etc. as photoconductive materials for use in electrophotographic photosensitive members. In contrast to many advantages thereof, such as, for instance, chargeability in a dark to a suitable potential, a little dissipation of charge in a dark, and fast dissipation ability by light irradiation, these photoconductive materials have the disadvantages of lacking the film forming property per se with a very few exceptions such as amorphous selenium and of poor ability to retain the charge given to their surface.
On the other hand, a variety of organic photoconductive materials are known, including photoconductive polymers such as polyvinylcarbazole, polyvinylanthracene, N-acrylamidemethylcarbazole polymer (Japanese Patent Application Laid-Open No. 85337/1975), 6-vinylindols (2,3-6)quinoxaline polymer (Japanese Patent Application Laid-Open No. 93432/1950) and also photoconductive polymers sensitized with trinitrofluorenone (U.S. Pat. No. 3,484,237) or with 2-aza-9-fluorenone (Japanese Patent Application Laid-Open No. 71236/1973), which, however, cannot be said so useful in practice since they generally have neither enough sensitivity for actual uses nor a sufficient good film forming property. There are also known low-molecular organic photoconductors such as pyrazoline compounds disclosed in British Pat. No. 1030024 and U.S. Pat. No. 3,180,729 and styryl compounds disclosed in Japanese Patent Application Laid-Open No. 94828/1976 and British Pat. Nos. 1465141 and 1465142. These low-molecular organic photoconductors have solved the problem of film forming property, which has been an objection to the application of the above organic photoconductive polymers, by selection of a suitable binder, but involve still many difficulties in practical applications because of low sensitivity of most of these organic photoconductor.
In view of the above, a photosensitive member of laminate structure has been recently proposed which comprises two photosensitive layers, a charge generation layer and a charge transport layer, having allotted functions. The electrophotographic photosensitive member having such photosensitive layers of laminate structure has been improved in sensitivity to visible light, in charge retentivity, and in surface strength.
Such a photosensitive member is provided with a laminate structure of photosensitive layers such as, for example, the following upon its conductive layer:
(1) a charge generation layer containing a methine dye derived from squaric acid or Diane Blue and a charge transport layer containing a pyrazoline compound (U.S. Pat. Nos. 3,824,099 and 3,837,851 and British Pat. No. 1453024);
(2) a charge generation layer comprising a vacuum-deposited polycyclic quinone pigment and a charge transport layer containing a polyarylalkane compound (German Patent Offen. No. 2929518);
(3) a charge generation layer comprising a vacuum-deposited a perylene pigment and a charge transport layer containing 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole (U.S. Pat. No. 3,871,882);
(4) a charge generation layer containing a disazo pigment and a charge transport layer containing a spiropyrazoline compound (Japanese Patent Application Laid-Open No. 112637/1979), N-(4-dialkylaminophenyl)carbazole (Japanese Patent Application Laid-Open No. 119925/1979), or a diarylmethane compound (Japanese Patent Application Laid-Open No. 108667/1980);
(5) a charge generation layer containing a cyanine dye and a charge transport layer containing 1,3,4-oxadiazole derivative (Japanese Patent Application Laid-Open No. 121742/1979); and
(6) a charge generation layer containing a methine dye derived from squaric acid or a disazo pigment and a charge transport layer containing a hydrazone compound (U.S. Pat. Nos. 4,150,987, and 4,278,747).
However, electrophotographic photosensitive members employing these compounds in the charge transport layer still do not have sufficient sensitivity and result in variations in surface potential particularly an increase in light portion potential and a decrease in dark portion potential, upon repeating charge and exposure. Additionally, these photosensitive members gain small quantities of charge at the surface upon corona charging and inferior in photomemory property.